The evolution of microelectronics circuit design and fabrication has yielded components of increased signal processing complexity and speed and which occupy a continuously decreasing (higher circuit density) circuit volume. Packaging of an appreciable variety of (low frequency) integrated circuit components (e.g. microprocessors, memories) has been effected through the use of in-line packages (dual or single) in which a microchip is encapsulated in a plastic material and interfacing to the circuitry of the chip is accomplished via a set of leads or pins that protrude from the sides of the package for engagement with a printed wiring board. For high frequency (microwave) applications, the unique dimensional tolerances and heat transfer characteristics of the devices have effectively prevented a standardized packaging scheme; instead microwave device housings have been typically custom designed, thereby contributing significant cost to production overhead. This custom configuration approach constitutes a significant drawback to the considerable strides that have been made in advanced materials technology such as gallium arsenide devices that have been employed for monolithic microwave integrated circuit devices.
One proposal to standardize the packaging of hybrid or monolithic microwave integrated circuits has been to house the circuit components in conventional T0-style plug-in cans, initially employed for discrete devices and in the early days of integrated circuit development. Because T0-style plug-in cans are part of an established packaging technology, reflecting specifications that have been standardized, they represent an attractive substitute for the costly custom designed housing configurations used for most microwave applications. Unfortunately because the conventional T0-style can was originally designed for packaging components that operate at frequencies considerably less than the signal processing bandwidth of present day microwave (e.g. GaAs-resident) devices, it has a practical upper frequency limitation on the order of 1 to 3 GHz or less.
A basic cause of this performance limitation is the dimensional parameters and materials of the plug-in leads (pins) employed in conventional T0-style cans. To meet structural rigidity, physical handling and plug-in receptor requirements of industry-standard specifications (e.g. 100 mil center-to-center pin spacings), a typical T0-8 type can may employ 18 mil diameter pins embedded in to 60 to 80 mil outer diameter glass seals, which yield a characteristic impedance Zo on the order of 35 ohms. For low frequency applications (UHF and below) the impedance mismatch (35 ohms of the pin/glass interface vs 50 ohms of the MMIC ports) presented to the signal coupling ports of the circuit components within the can does not dramatically affect circuit performance. For the considerably higher frequency range (10-40GHz) over which present day microwave integrated circuit devices are designed to operate, the impedance mismatch of the pin/glass interface of the T0-can introduces such a large and prohibitive signal coupling loss that it cannot be used, thereby necessitating housing the circuit in a custom designed package.